Posttranslational Processing of Infected Cell Protein 22 Mediated by Viral Protein Kinases Is Sensitive to Amino Acid Substitutions at Distant Sites and Can Be Cell-Type Specific

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Abstract
Infected cell protein 22 (ICP22) is posttranslationally phosphorylated by the viral kinases encoded by U S 3 and U L 13 and nucleotidylylated by casein kinase II. In rabbit and rodent cells and in primary human fibroblasts infected with mutants from which the α22 gene encoding ICP22 had been deleted, a subset of late (γ 2 ) gene products exemplified by U L 38 and U S 11 proteins are expressed at a reduced level, as measured by the accumulation of both mRNA and protein. The same phenotype was observed in cells infected with mutants lacking the U L 13 gene. The focus of this report is on three serine- and threonine-rich domains of ICP22. Two of these domains are homologs located between residues 38 to 66 and 300 to 328. The third domain is near the carboxyl terminus and contains the sequence T374SS. The results were as follows. (i) Alanine substitutions in the amino-terminal homolog precluded the posttranslational processing of ICP22 in rabbit skin cells and in Vero cells but had no effect on the accumulation of either U S 11 or U L 38 protein. (ii) Alanine substitutions in the carboxyl-terminal homolog had no effect on posttranslational processing of ICP22 accumulating in Vero cells but precluded full processing of ICP22 accumulating in rabbit skin cells. The effect on accumulation of U L 38 and U S 11 proteins was insignificant in Vero cells and minimal in rabbit skin cells. (iii) Substitutions of alanine for the threonine and serines in the third domain precluded full processing of ICP22 and caused a reduction of accumulation of U S 11 and U L 38 proteins. These results indicate the following. (i) The posttranslational processing of ICP22 is sensitive to mutations within the domains of ICP22 tested and is cell-type dependent. (ii) Posttranslational processing of ICP22 is not required for accumulation of U L 38 and U S 11 proteins to the same level as that seen in cells infected with the wild-type virus. (iii) The T374SS sequence shared by ICP22 and the U S 1.5 proteins is essential for the accumulation of a subset of γ 2 proteins exemplified by U S 11 and U L 38 and is the first step in mapping of the sequences necessary for optimal accumulation of U S 11 and U L 38 proteins.

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